1. Field of the Invention
The present invention relates to an actuator element using a piezoelectric thin film, and more particularly to an actuator element having a structure in which a diaphragm is displaced by piezoelectric driving, and to a method of driving such an element and a method of manufacturing such an element, and a device inspection method and a Micro Electro Mechanical System (MEMS) switch using such as actuator element.
2. Description of the Related Art
In recent years, switch elements having various structures and technology for manufacturing same have been proposed in the field of MEMS devices (see, for example, Japanese Patent Application Publication No. 2006-32339, Japanese Patent Application Publication No. 2009-136934, Japanese Patent Application Publication No. 2009-9884, Japanese Patent Application Publication No. 2009-245877 and Japanese Patent Application Publication No. 2008-91167). Japanese Patent Application Publication No. 2006-32339 proposes an rf (Radio Frequency)-MEMS switch which is driven by an electrostatic method. Japanese Patent Application Publication No. 2006-32339 discloses technology for preventing charge accumulation and a consequent sticking phenomenon (sticking of the element), by providing a semiconductor layer between electrodes. Japanese Patent Application Publication No. 2009-136934 proposes a method of manufacturing a MEMS device including a step of removing a sacrificial layer using an electrical corrosion reaction employing aqua regia, from the viewpoint of preventing sticking as a result of residual etching solution during manufacture.
Japanese Patent Application Publication No. 2009-9884 proposes a method of manufacture in which a sacrificial layer provided between a movable electrode and an attracting element is divided into a first sacrificial layer and a second sacrificial layer made of materials of respectively different types, in order to resolve a problem of sticking which occurs in a process for manufacturing a MEMS switch which is driven by an electrostatic force method. Japanese Patent Application Publication No. 2009-245877 discloses a composition in which an insulating film for preventing sticking is formed on the front surface of a base substrate on which a fixed electrode is formed.
Japanese Patent Application Publication No. 2008-91167 proposes a method of resolving problems of sticking by using a c-axis oriented film of aluminum nitride (AlN) or zinc oxide (ZnO) instead of using a lead material in the piezoelectric film.
If an actuator using a piezoelectric film made of a lead material (PZT material) is described on the basis of the description in Japanese Patent Application Publication No. 2008-91167, then in the related art, such an actuator has been driven using a piezoelectric body having characteristics such as those shown in FIG. 6 and FIG. 7. FIG. 6 is a diagram showing the relationship between the electric field E applied to a ferroelectric film and the polarization Pr, which is known as the Pr-E hysteresis curve. FIG. 7 is a diagram showing the relationship between the distortion (electrostriction) c produced in a ferroelectric body and the applied electric field E. These diagrams are cited from FIG. 4 and FIG. 5 of Japanese Patent Application Publication No. 2008-91167.
In the case of a switch element (for example, an element having a cantilever structure) which uses a piezoelectric body having the characteristics shown in these drawings, the displacement in the opposite direction is not obtained even if a voltage in the opposite direction is applied to the piezoelectric body, and a drive force in a separating direction which resolves the issue of sticking is not obtained.
In other words, the hysteresis characteristics in FIG. 6 have left-right symmetry with respect to positive and negative electric fields. Furthermore, in FIG. 7, if the sign of the applied voltage (the direction of the electric field) is changed, then although the distortion (displacement) acts slightly in the opposite direction, effectively, the distortion (displacement) only acts in the same direction even if the sign of the applied voltage (direction of the electric field) is changed. Consequently, sticking is not prevented (resolved), even by driving with application of an opposite voltage.
In this respect, Japanese Patent Application Publication No. 2008-91167 discloses using a piezoelectric body made of AlN or ZnO instead of a lead-based (PZT based) piezoelectric body, but the piezoelectric constant of AlN or ZnO is 1/10 or less of the piezoelectric constant of the PZT based, and therefore the force produced by driving the actuator is very weak. Therefore, the method described in Japanese Patent Application Publication No. 2008-91167 is not sufficient to prevent sticking.